In the oil industry slips have been essential components of oil field drilling and servicing equipment for many years. Conventional slips are sets of heavy hinged blocks with gripping dies that are positioned in a slip bowl of a rotary table to engage tubing, such as drill pipe, casing or production tubing suspended in a wellbore. Angled surfaces in each slip block mate with angled surfaces in the slip bowl. The angled surfaces cause axial forces exerted on the slip blocks by the weight of the tubing to be transferred into lateral gripping pressure on the tubing. The gripping pressure supports the tubing and prevents it from slipping down through the slips into the wellbore.
As is well known in the art, conventional slips are manually engaged by oil field personnel who maneuver the slips into the slip bowl so that they slide into engagement with a casing, drill or production tubing pipe. The slips are disengaged by upward axial movement of the casing, drill pipe, or production tubing to remove weight from the slips. The slips are then lifted out of the slip bowl. An example of such conventional slips is described in U.S. Pat. No. 4,244,093, entitled TURBINE SLIP PULLING TOOL, which issued to Klingsensmith on Jan. 13, 1981.
There is an ever increasing demand for producing more oil and gas from existing wells. After a primary recovery term of a well has expired, some form of reworking is required to produce at least a portion of the remaining oil and/or gas from the well. In reworking a well, such as in preparation for a well stimulation process, the tubing string must be removed from the well or pulled up to permit the tubing hanger to be removed so that stimulation fluids can be pumped down through an annulus between the production tubing and the casing. During such operations the tubing string is supported as required, by slips. It is therefore necessary to set and remove the slips during preparation for a well stimulation process. Consequently, slips are not only frequently used during well drilling and completion, they are also essential equipment for well re-completion, servicing and workover.
It has been increasingly apparent that well serving and workover are best performed under “live well” conditions. A live well is a well in which downhole pressure are controlled by wellhead equipment. As is well known, slip assemblies generally do not provide pressure seals to inhibit the escape of hydrocarbons from the well. Consequently, the use of slip assemblies over a live well generally requires either the use of hydril blowout preventers in conjunction with ram-type blowout preventers, to control well pressures unless the well is “killed” by pumping in a overbearing fluid, such as drilling mud to prevent fluids from escaping from the well. Either option contributes significantly to treatment costs. Each option also has other disadvantages. For example, killing a well can reverse the beneficial effects of a well stimulation process. On the other hand, the use of one or more hydril blowout preventers significantly raises working heights, making the well more difficult to work and compromising worker safety.
There therefore exists a need for a pressure containing slip spool that integrates into a wellhead control stack to overcome the shortcomings of the prior art slip assemblies, while being robust and reliable enough to support even very long strings of coiled or jointed tubing.